Brain damage and/or death of a subject are direct consequences of prolonged apnea (absence of respiration), particularly when combined with hypoxia. Diagnosing apnea and hypoventilation (a reduction in breathing rate) are of particular concern when a patient is sedated, or when their breathing fails during sleep (as in obstructive sleep apnea).
Identifying hypoventilation before apnea occurs by monitoring breathing frequency and/or breath volume of the patient allows time to intervene to stimulate breathing, relieve airway obstruction, and restore effective respiration of the subject before hypoxia occurs. Hypoxic-ischemic brain damage may result from inadequate or loss of respiration, which if not reversed early in the development, may lead to brain damage and possible death. There are numerous human tragedies of such consequences from a failure to adequately monitor respiration.
Generally, respiratory rate can be monitored continuously and non-invasively by measuring the carbon dioxide partial pressure in the exhaled breath. However, carbon dioxide measurements do not change directly with breath volume, only rate. No portable, disposable and inexpensive carbon dioxide detectors are known in the prior art.
Ventilation may also be measured by examining displacement of the chest and rib cage using sophisticated monitors found in operating rooms and in sleep laboratories. However, such monitors are non-transportable, non-portable, cumbersome to use, generally expensive and are not widely available.
At the present time, it is not believed that there are any devices that provide for non-invasive/portable/measures of ventilation in patients whose airways are not instrumented in some way.
Such a monitoring system is needed to detect the presence of breathing, to measure respiratory rate and to estimate the breathing volume in both patients in hospitals and others outside hospitals, in doctors' offices and in the community (firemen, miners and armed forces).
In addition, the limits of performance of such a respiratory monitor must address its application in trauma which may occur outside healthcare institutions, in deserts, fires and mines. Hence the device must be effective when applied internal or external to an enclosure or mask over the patient's face, and when increased or decreased ambient temperatures are present. Such temperature changes in the local environment could limit the performance of the monitor for which there must be compensation. Thus, a system is needed that provides for consistent and reliable monitoring of respiration that is independent of the ambient temperature.